Trying to make sense of why Otis exploded en route to Acapulco this week

One-sentence summary

Scroll to the bottom for a couple notes on current weather, but in today’s post, I want to try to make some sense of what just happened in Mexico this week.

Otis’s catastrophic rapid intensification: What happened?

In general, I think Eric did a good job yesterday sort of conveying the general shock many of us in the meteorology community had with regard to Hurricane Otis. The word “unprecedented” gets tossed around a little too fast and loose these days, but truly, this was without any real precedent in Acapulco. And it was with only slight precedent anywhere in terms of how quickly it intensified.

Courtesy of Dr. Kim Wood, University of Arizona

Otis was the textbook definition of rapid intensification (RI), going from a 50 mph tropical storm on Monday evening to a 165 mph category 5 hurricane last night. Look at the change in Otis between Sunday evening and Tuesday evening, 48 hours apart. You can see the intensification chart to the right of the satellite loop. Through about mid-morning on Tuesday, everything was going basically as you’d expect for a modest hurricane with Otis. It may have been tracking toward a category 2 type landfall, or even a category 3 type landfall in a worst case, if you assumed the general rules of RI in this region.

Much like an onion, there are layers to this story that are important. First, take it from one of the more seasoned hurricane hunters, this was not what they expected when they flew their mission on Tuesday.

Jeremy DeHart on the platform formerly known as Twitter

And this was before Otis had peaked. The typical satellite-derived intensity values often used to “proxy” intensity of storms that are far away from reconnaissance flights failed in this case to grasp how intense Otis was. In other words, Otis intensified so quickly that it basically outran the ability to measure how intense it actually was.

Here was the raw model output for Otis from Tuesday morning. This is what general weather forecasters would use to assess what the models assume will happen with a storm’s wind forecast. The dashed line is what actually occurred.

Otis’s model forecasts on Tuesday early morning were nowhere remotely close to what happened. (Tomer Burg on Twitter)

None of the best, most reliable tropical modeling had Otis as a hurricane, let alone a category 5 storm. To put it bluntly, this was an absolutely catastrophic forecast failure.

Even the National Hurricane Center by late Tuesday morning had it at 90 mph making landfall, well above any forecast data, and they concluded in their discussion that it seemed reasonable to potentially see further intensification adjustments before landfall. So even in the worst case scenario, the NHC forecast would have still been off by probably two categories less than 18 hours before landfall. And this was using strong meteorological analysis to bias correct the models upward too. And to their credit, they had it at 140 mph by the late afternoon advisory.

Interestingly, one of the tools we use to forecast the probability that a storm will rapidly intensify, SHIPS guidance, failed also. Early on Tuesday morning it only showed about a 2-3x above normal chance that the storm would intensify from a 50 mph tropical storm to a 100-125 mph hurricane. Yes, that is above climatology, but it’s not exactly anything impressive given what we’ve seen in recent years.

By Tuesday afternoon, those odds had increased to 5-9x above normal. But even this only showed 2x above normal odds that we’d get to 140 mph+. There were finally some hints available by mid to late morning on Tuesday, but nothing that would have offered a meaningful forecast improvement over what the NHC had (which called for 20-30 mph of intensification over 12-24 hours).

SHIPS rapid intensification forecast guidance from Tuesday midday showed about 9x greater than normal probability of 50 mph of intensity gain, which at that point would have meant a category 2 or 3 storm. The odds of something truly massive were still only about 2x above normal. (UCAR)

So the first question is, why did Otis do what it did? Probably a combination of a couple things. First, Otis was placed ideally in an environment that facilitated constructive wind shear. When we discuss wind shear it’s usually referenced in a negative sense; wind shear inhibits and destroys storms. Well, in occasional cases, as we’ve witnessed in the Gulf with Ian, Delta, Zeta, among many other storms in recent years, the wind shear can actually be constructive and help ‘vent’ the system. In this case, Otis was optimally placed in the right entrance region of the jet stream.

An analysis map from the GFS model showing jet stream winds on Tuesday morning. Otis (circled) is tenuously placed in the right entrance region of the jet stream, which tends to offer a more favorable environment for intensification. (Tropical Tidbits)

Storms are aided in intensification when placed in the left front or right rear (entrance) of the jet stream. Why? In that portion of the jet stream, the winds aloft diverge, meaning they either are moving in opposite directions or stronger wind is diverging away from weaker wind. Upper level divergence leads to rising air. Rising air is necessary for storms to form and maintain, and thus surface pressures tend to fall in this region of the jet stream as well.

A conceptual model of a jet streak showing that the right entrance region (bottom left quadrant) is supportive of upper level divergence and surface pressure falls (Penn State University)

Otis wasn’t exactly square in the middle of the right entrance region, but it was there. Additionally, Otis tracked right over an area of 31°C sea surface temperatures.

Sea surface temperatures were close to 31°C over Otis’s path, which is a few degrees above normal. While not the primary cause of Otis’s rapid intensification, they did not hurt. (Brian McNoldy on Twitter)

This rich warm water did not hurt matters at all.

So how did every reliable model we use miss this? That’s for graduate students and researchers to answer in the coming years, because I have no formal idea. There was something about Otis that models just could not capture and translate to RI. Human forecasters, recognizing the setup were able to mitigate some but not all of the underforecast issue. Otis’s smaller size may have also contributed. My hunch is also that if Otis had tracked, say 30-40 miles east or west of where it was, it would not have gone off to the races like this. It was simply perfectly placed to optimize intensification.

A less sexy explanation? Otis was almost unprecedented in the historical record. Only Hurricane Patricia in the Pacific in 2015 had a greater rate of RI than Otis. And in the modern era no massive storm has hit Acapulco.

Otis was not the fastest rapidly intensifying storm on record, but it was near the top of the list in the East Pacific, lagging only 2015’s Patricia. (Tomer Burg on Twitter)

Unprecedented outcomes are just that, and if the historical record has only one other remotely comparable event in this region, it becomes tough to expect that modeling can “capture” the concept that this would occur in this environment.

It feels like this was a combination of bad luck, bad timing, and bad placement. And it just so happened that a metropolitan area with over 1 million people was in the way.

It’s easy to sit here and pontificate about this or to say that weather forecasts are often wrong. But the thing is, they’re not. They’re often right. With hurricanes, forecasting has improved by leaps and bounds in the last 20 to 30 years. Perfect? No, but often more than acceptable. A failure like this shocks us because we aren’t actually used to forecast failures of this magnitude anymore! 100 years ago? Sure, this was fairly routine. But in the 2020s, we have standards and expectations for weather forecasts, and clearly this was a shock to the system. Busts like this remind us that it’s an imperfect business and there is still much work to do. The work to be done to understand Otis will take time, but we will certainly see many research papers in the coming years.

Weather update: Tammy, Texas rain, and continued wintry weather north

Just a quick punch list of items we’ll be watching over the next few days.

Hurricane Tammy’s final advisory was issued this morning, as it has transitioned into an extratropical entity, or something akin to a strong nor’easter. It will meander east of Bermuda for a few days, and there is some chance this could reacquire tropical characteristics at some point. It won’t go quietly, but hopefully any impacts to Bermuda are minimal.

Post-tropical Tammy will remain a strong storm for several more days as it meanders east of Bermuda. (Tomer Burg)

Flash flooding concerns will be in the forecast in the southern Plains over the next few days, meandering from Texas into Arkansas. Generally 3 to 5 inches of rain is possible in spots. But higher amounts are a virtual certainty. South of Dallas, the 24 hour rainfall totals in Hood and Somervell Counties are estimated to be close to 10 inches.

Rain totals southwest of Dallas and Fort Worth are estimated to be close to 10 inches (purple) over the last 24 hours. (NOAA NSSL)

Numerous flash flood warnings are posted in North Texas and Hill Country this morning as heavy rain falls and spreads south and east.

Snow will continue in the northern tier. Over a foot fell in Helena, Montana, with snow spreading into North Dakota. Snow should wind down tonight from west to east.

A bunch of items will impact the interior U.S. over the next 3 to 7 days, ranging from heavy snow in the Rockies to heavy rain in the mid-Mississippi Valley to hazardous cold in the North. (NOAA WPC)

The hazards map from the Weather Prediction Center is lit up for days 3 to 7 with cold infiltrating from Canada this weekend in the North, the aforementioned heavy rain in mid-Mississippi Valley, and frost and freeze risk from near the Permian Basin and southern New Mexico across to the Ohio and Tennessee Valleys. We’ll have more tomorrow!

October 25, 2023 Outlook: After a shocking burst of intensification, Hurricane Otis makes a catastrophic landfall near Acapulco

One-sentence summary

We are going to talk about Otis today because of the hurricane’s dramatic intensification on Tuesday, and overnight landfall along the Southern Pacific coast of Mexico.

Rapid intensification

Otis made landfall on Tuesday night, near Acapulco, with maximum sustained winds of 165 mph. This was a worst-case scenario for this region. Why? Because no storms had been recorded in this area, this strong, before. On top of that, local residents and business owners had less than a day to prepare for the worst hurricane of their lives. So not only was this storm unexpected, there was no institutional memory about what to expect from a major hurricane.

Hurricane Otis nears the southern coast of Mexico on Tuesday. (NOAA)

With this startling burst in intensity, Otis has nearly set a record for rapid intensification within 24 hours. The system strengthened from a 50 mph tropical storm at 1 am CT on Tuesday to 165 mph just 23 hours later. That is 115 mph in 24 hours. It is second only to Hurricane Patricia, a Pacific storm in 2015 that saw its maximum sustained winds increase by 120 mph during a similar period.

This morning, as it moves inland, Otis is weakening. As of 7 am CT the storm had 110 mph winds, and will continue to lose intensity as it interacts with mountainous terrain. Nevertheless, Otis will continue to bring damaging winds into Southern Mexico today, along with dangerous storm surge. Heavy rains will remain a problem later this week, through Thursday, for much of Southern Mexico. They are likely to produce significant flooding and mudslides.

Forecast track for Hurricane Otis. (National Hurricane Center)

Completely blind-sided

Let’s wind things back to Monday night, and have a look at the model forecasts for the intensity of Hurricane Otis. At the time this was a tropical storm, and largely expected to remain so before its landfall into Mexico. None of our ‘best’ models for predicting tropical system intensity anticipated Otis growing beyond tropical storm-strength. In two decades of forecasting I do not recall a whiff like this one.

A plot of intensity forecasts for Hurricane Otis. (Tomer Burg, Blue Sky).

As a meteorologist, these kind of moments are humbling. Otis will be studied in the coming months and years to understand why it blew up so quickly, and so powerfully, in such a short period of time. In moments like these, forecasters utterly failed the people of Southern Mexico. We must do better.

Hurricane Tammy

Over in the Atlantic Ocean, Hurricane Tammy continues to dance around the Atlantic Ocean. This storm would be a curiosity given its meandering track, but for its potential to come near Bermuda this weekend. As of Wednesday morning, Tammy has sustained winds of 100 mph, and there is a chance for some slight strengthening today.

However, after today it is likely to interact with a cold front, and begin a transition to a non-tropical storm. There is a fair amount of uncertainty in track and intensity. But for now it looks like Tammy will remain far enough south, and just weak enough, to not bring anything more than garden-variety like storminess to Bermuda this weekend.

Tammy is going to have a walk-about in the Atlantic this week. (National Hurricane Center)

Beyond Tammy, happily, the Atlantic tropics look quiet.

October 24, 2023 Outlook: Otis should impact Mexico as a hurricane, while a significant winter storm impacts the northern U.S. and southern Canada

One-sentence summary

Tammy continues to provide some uncertainty, and Otis will come ashore near Acapulco as a hurricane tomorrow, while the U.S. sees its first big winter storm in the North that will help usher in a round of colder air.

Tropics: Tammy being tricky, while Otis approaches Acapulco tomorrow

Hurricane Tammy continues to plod along in the southwest Atlantic moving northeast just under 10 mph. It continues to take on a less than classic look for a hurricane, but it is still maintaining hurricane intensity.

Hurricane Tammy certainly looks a little ragged, but it is expected to intensify a little more before beginning to transition into an extratropical system. (Weathernerds.org)

Tammy still has just a handful of showers trailing it into the islands, but otherwise, impacts from Tammy are limited to rough surf and rip currents at this time. The forecast track for Tammy continues to fan out all over the Atlantic, as various features that will dictate Tammy’s future track remain a bit uncertain.

There remains significant uncertainty on Tammy’s future track in the Atlantic. (Tomer Burg)

This is one of the sloppiest tropical system forecasts I’ve seen this year. The 180° spread in options is one of my least favorite ones to manage during hurricane season. Thankfully, the stakes are not as high in late October usually, and in this instance there is good agreement among the models that Tammy will slowly weaken such that most impacts to either Bermuda or the Bahamas and Florida would be modest at worst. We will keep an eye on things regardless, but at this time, Tammy is not an overly serious concern down the line.

Meanwhile, in the Eastern Pacific, Otis is likely to become a hurricane. Very few storms have not outpaced their forecast intensity this year it seems in the East Pac, and Otis will be another.

Otis is much healthier looking than Tammy as it approaches hurricane intensity and a likely landfall tomorrow near or west of Acapulco in Guerrero, Mexico. (Weathernerds.org)

Otis should become a category one hurricane tonight or tomorrow before making landfall in the state of Guerrero in Mexico, probably just west of Acapulco. Hurricane warnings are up for the coast of Guerrero, with tropical storm warnings extending southeast into a portion of Oaxaca.

Otis should track near to Acapulco tomorrow as it makes landfall as a hurricane. (NOAA NHC)

Beyond Otis, the Pacific should quiet down some.

Elsewhere in the tropics, things look fairly quiet. Tropical Depression 21 formed near Nicaragua yesterday and is ashore this morning, producing areas of heavy rain for Central America. There are no other serious things to watch at this time heading into the rest of the week.

U.S. Weather: Significant northern tier winter storm will usher in colder air that eventually spreads south and east

Winter Storm watches and warnings are posted from Washington across Montana and into North Dakota today as the first real big winter storm of the season gets going in the U.S.

Winter Storm watches & warnings are posted for the northern Cascades and much of Montana (extending into North Dakota) as the season’s first major winter storm kicks into gear. (Pivotal Weather)

Snow and cold temperatures will kick into gear across central and western Montana, with 8 to 12 inches of snow possible for places like Great Falls.

Significant snow is likely across interior Montana later today and tomorrow, along with gusty winds. As much as 8 to 12 inches is expected over a rather large swath of the region. (NWS Great Falls)

Over the next 3 days, snow will likely spread into North Dakota, and the current 50th percentile forecast for the next 72 hours will be 8 inches or higher there

Snow of 8 inches or more should spread across North Dakota and into southern Manitoba. (NOAA WPC)

Snow should expand north into southern Manitoba and southeast Saskatchewan as well. Travel will be pretty rough across the northern tier over the next couple days. The first significant storm of the season always usually causes extra travel headaches too, so if you plan to be up north at all this week, take it easy.

It will turn quite chilly behind this storm over the next few days, with much below average temperatures across Montana and Alberta. Cool temperatures will expand into the Dakotas and much of the West Coast as well.

Cold weather will take hold behind this storm in the northern Rockies and Cascades, as well as in much of southwest Canada. (Weather Bell)

A handful of record lows are possible up that way. Meanwhile, downstream from the storm, extremely warm temperatures will dominate with numerous record warm low temperatures expected over the next few days.

Beyond this, a more potent south and east push of colder air is likely next week, erasing most of the warm weather for now in the Central and Eastern U.S.

An aggressive push of colder air is likely in the Plains and portions of the Upper Midwest next week, with cool temps expanding south and east from there. (NOAA CPC)

Halloween should be a cooler one in much of the Central and Eastern U.S.

October 23, 2023 Outlook: Tricky Tammy, Otis in the Pacific, and some heavy rain in the Southern Plains

One-sentence summary

Hurricane Tammy will take an unconventional track over the next few days, but it is expected to peak in intensity before weakening by the time it could maybe threaten land again.

Atlantic tropics: Tammy, Tammy, Tammy

Tammy has shifted north of the islands now, and it is unlikely to directly impact any land significantly for the foreseeable future. That said, there are still a lot of questions with Tammy’s future track. For one, how far north will it get before it turns west? This will have implications for Bermuda. And for how long will it maintain tropical characteristics?

Tammy is expected to track northeast and intensify the next day or two before running into some issues and hooking back west somewhere southeast of Bermuda. (NOAA NHC)

For now, the general thought is that Tammy will move north and northeast over the next couple days and strengthen some before slowing down and cutting back west while losing tropical characteristics. Some models do continue to push Tammy north and northeast out of the picture entirely, so there is a pretty healthy degree of uncertainty still. If this were September, I might be more intrigued and concerned by this, but given how late in the season it is, the stakes are probably not as high. Modeling is in good agreement that Tammy will peak in intensity early this week before weakening. Ultimately, we can probably just observe Tammy from afar without much more than bemusement at this point. That said, interests in Bermuda, the Bahamas, and Florida probably want to check back in on this through the week to make sure all is going to plan.

In the meantime, you can see some heavy thunderstorm activity remaining over the northeast Leeward Islands this morning. That should subside this afternoon as Tammy puts more distance between itself and the islands.

Tammy has plenty of moisture and thunderstorms with it, but it is definitely not the most classic looking hurricane. (Weathernerds.org)

Elsewhere in the Atlantic, Invest 95L is right off the coast of Central America and it has the potential to become a short-lived depression or storm before it moves inland tomorrow. In the interim, 3 to 6 inches of rain could fall in Nicaragua over the next day or two leading to some flash flooding.

Rain totals in Central America will be on the order of 2 to 6 inches, with the highest amounts in eastern Nicaragua. (Weathernerds.org) Editor’s note: I accidentally mislabeled Costa Rica as Panama in this image. My apologies!

Beyond Tammy and 95L, we don’t expect any other development in the Atlantic this week.

Pacific tropics: Mexico may notice Otis

Meanwhile, the Pacific is calming down a bit, but not before we deal with Tropical Storm Otis. That one is not currently expected to grow beyond tropical storm intensity as it tracks north and northwest toward the coast of Mexico this week.

Tropical Storm Otis is expected to make landfall later this week on the coast of Mexico near-ish to Acapulco. (NOAA NHC)

It’s expected to come in probably in Guerrero, not far from Acapulco. We’ll keep an eye on this through the week, but as of now, tropical storm watches are posted for a chunk of the Mexico coast.

Texas and Southern Plains rain

Just a quick closer today on the rainfall expected this week in the Southern Plains. Portions of west Texas up into Oklahoma are in the “slight” risk for excessive rainfall today and tomorrow (level 2 of 4). Locally heavy rain will occur in central and west Texas up into southern Oklahoma the next couple days.

Locally heavy rain may cause localized flash flooding in portions of west or central Texas and Oklahoma this week. (NOAA WPC)

Total rainfall over the next 3 days could exceed 4 inches portions of this area.

Anywhere from 2 to 5 inches of rain is possible from the Permian Basin of Texas into Oklahoma over the next few days. (Pivotal Weather)

For the most part this will be welcome rainfall in these areas.

Tomorrow, we’ll talk more about snow from Washington into Montana, as well as the potential for some chilly air to invade later this week and next week.